When feeding wood chips, for instance, into a continuous digester a certain minimum quantity of liquid must be added through the supply system together with the chips. If too little liquid is added in relation to the quantity of chips the temperature will become too high in the supply system and the sinking rate of the chips in the impregnating vessel or digester will be disturbed. The requisite amount of liquid is normally obtained with the digesting liquor, e.g. white liquor, added to the supply system for impregnating the chips. If necessary spent digesting liquid (black liquor), the temperature of which has been lowered through relief in several steps or by means of indirect cooling, may be returned to the supply system, thus increasing the amount of liquid available. In certain multi-step processes in which the first step is impregnation or digestion with a different chemical from that used in subsequent digestion step or where for technical reasons it is unsuitable to use fully spent digesting liquid (black liquor), the requirement for a certain minimum quantity of liquid must be fulfilled by adding a sufficient amount of the digesting liquid from the first step, possibly diluted with water. This entails several drawbacks such as poorer heat economy due to the increased consumption of steam in subsequent cooking steps and/or increased need for evaporation for recovery of the chemicals. Another drawback may be that the concentration of the cooking chemicals in first step becomes too low to achieve the desired effect of the process. Addition of black liquor to the supply system is described in Se 359 331 and U.S. Pat. No. 5,080,755. In the processes according to said documents liquid is withdrawn from the impregnation system at a point located at a predetermined distance from the inlet end, and the liquid withdrawn is mixed with black liquor.
The liquid to wood ratio at the inlet to a continuous digester is normally 3.0-3.5 m.sup.3 per ton dry chips. In conventional sulphate cooking 70-100% of the white liquor required is normally added to the supply system for the impregnating vessel via a high-pressure pump. The amount of liquid at the inlet to the impregnating vessel may then be within an interval of about 2.4-2.8 m.sup.3 per ton dry chips, distributed as follows:
______________________________________ A Chips moisture (moisture content 50%) 1.0 m.sup.3 B + C Steam condensate 0.3 m.sup.3 E White liquor (70-100%) 1.1-1.5 m.sup.3 ______________________________________
As mentioned above, black liquor can be added to the impregnating vessel through the supply system in order to increase the liquid to wood ratio, for instance to 3.0 m.sup.3 or above. The temperature of the black liquor should not exceed 100.degree. C. If too little liquid is added to the impregnating vessel, disturbances will result in the supply system due to the formation of steam in the down pipe located before the high pressure valve. The temperature of the liquid exceeds the temperature corresponding to the steam pressure in the pressure steaming vessel located upstream. Sinking of the chips in the impregnating vessel is also impeded. The quantity of free liquid, i.e. the portion of liquid which is not absorbed by the chips, becomes too low and the flow rate of the liquid is correspondingly reduced. The flow rate of the free liquid exerts a driving force on the column of chips. It is important that this driving force is great in the upper part of the impregnating vessel where the chips are still not saturated with liquid but have a tendency to float. This requirement is of less importance in the lower part of the impregnating vessel since by this stage the chips are substantially saturated with liquid and therefore tend to sink due to their own weight. When treatment is carried out in several steps using different digesting liquids in the various steps, there is the same need to maintain a sufficiently high liquid to wood ratio in the first step, i.e. in the initial concurrent flow zone in the impregnating vessel. However, it may be that the process or operating economy requires that the digesting liquid of the first step has to be added in a limited quantity, in which case it may be difficult to achieve a sufficiently high liquid to wood ratio. At the same time, the liquid flow at the entrance to the digester must be controlled so that it does not become too great since this results in too great a consumption of high-pressure steam in heat exchanger present in the transfer system and of direct steam to the top of the digester. Furthermore, the withdrawal strainer in the upper part of the countercurrent flow washing zone of the digester becomes overloaded.